1. Field of the Invention
The present invention relates to an image recording medium that is capable of recording image information as an electrostatic latent image.
2. Description of the Related Art
For example, in medical X-ray photographing, for the purposes of a reduction in the radiation dose to which a subject is exposed, an enhancement in diagnosis performance, etc., there has been disclosed a system that uses an image recording medium employing a photoconductor sensitive to X-rays (e.g., a selenium (Se) plate, etc.,). In the system, an electrostatic latent image is recorded on the image recording medium by the use of X-rays and then the electrostatic latent image is read out (e.g., U.S. Pat. Nos. 4,176,275, 5,268,569, 5,354,982, 4,535,468, xe2x80x9c23027 Method and Device for Recording and Transducing an Electromagnetic Energy Pattern,xe2x80x9d in Research Disclosure, June 1983, Japanese Unexamined Patent Publication No. 9(1997)-5906, U.S. Pat. No. 4,961,209, xe2x80x9cX-ray Imaging Using Amorphous Selenium,xe2x80x9d in Med. Phys. 22(12), etc.).
The above-mentioned U.S. Pat. No. 4,535,468 discloses an image recording medium, in which (1) a 100xcx9c500-xcexcm-thick photoconductive recording layer with amorphous selenium (xcex1-Se) as its main component, (2) a 0.01xcx9c10.0-xcexcm-thick intervening layer (trap layer), which consists of AsS4, As2S3, As2Se3, etc., for storing, as a trap, a latent-image polarity charge generated within the photoconductive recording layer, (3) a 0.5xcx9c100-xcexcm-thick photoconductive reading layer with amorphous selenium (xcex1-Se) as its main component, and (4) a 100-nm-thick reading-light transmitting electrode layer, consisting of Au or indium tin oxide (ITO), which allows reading electromagnetic waves (hereinafter also referred to as reading light) to pass through it, are stacked in the recited order on a 2-mm-thick recording-light transmitting electrode layer (conductive substrate), consisting of Al, which allows recording electromagnetic waves (hereinafter also referred to as recording light) to pass through it. Particularly, it is disclosed that the use of the reading-light transmitting electrode layer as a positive electrode is preferred because the satisfactory hole mobility of xcex1-Se can be utilized and that a blocking layer, consisting of an organic substance, is interposed between the reading-light transmitting electrode layer and the photoconductive reading layer in order to prevent S/N-ratio degradation due to the direct injection of electric charge through the electrode of the reading-light transmitting electrode layer. That is, this image recording medium is a multi-layer recording medium having both high dark resistance and excellent read response speed, and the image recording medium is constructed mainly of a layer having xcex1-Se as its main component.
For enhancing the S/N ratio of an image, and also for performing parallel reading (mainly in a horizontal scanning direction) to shorten the reading time, there are cases where the electrode of the reading-light transmitting electrode layer is replaced with a stripe electrode constructed of a large number of electrode elements (line electrode elements) disposed at intervals of a pixel pitch (e.g., Japanese Patent Application No. 10(1998)-232824 filed by the present applicant). However, in the stacked construction of the image recording medium described in above-mentioned U.S. Pat. No. 4,535,468, in the final fabrication step the reading-light transmitting electrode layer must be formed after formation of the photoconductive reading layer, so it is difficult to form the aforementioned stripe electrode. The reason for this is that photoetching is used for forming the electrode elements of the stripe electrode, but since a baking step is usually performed on photoresist at high temperature (e.g., 200xc2x0 C.), xcex1-Se forming a previously formed photoconductive layer cannot endure such a high temperature and therefore the characteristics will be degraded. Furthermore, since an alkaline developing solution, which is employed in the step of developing the photoresist, contacts xcex1-Se and gives off harmful gases, steps becomes complicated for removing the harmful gases, resulting in an increased cost.
On the other hand, the present applicant has proposed, in the aforementioned Japanese Patent Application No. 10(1998)-232824, an image recording medium (electrostatic recording body) in which (1) a recording-light transmitting electrode layer which allows recording light to pass through it, (2) a 50xcx9c1000-xcexcm-thick photoconductive recording layer with amorphous selenium (xcex1-Se) as its main component, (3) a charge transfer layer for forming a charge storage portion, which consists of xcex1-Se doped 10 to 200 ppm with an organic substance or Cl and stores a latent-image polarity charge generated in the photoconductive recording layer, at the interface between the charge transfer layer and the photoconductive recording layer, (4) a photoconductive reading layer with xcex1-Se as its main component, and (5) a reading-light transmitting electrode layer which allows reading light to pass through it, are disposed in the recited order. The aforementioned Japanese Patent Application No. 10(1998)-232824 does not disclose whether the image recording medium is fabricated in sequence from the recording-light transmitting electrode layer or conversely from the reading-light transmitting electrode layer. The image recording medium can be formed in either order. However, the aforementioned Japanese Patent Application No. 10(1998)-232824 has proposed that a conductive substance such as a film of NESA (SnO2) is provided as the reading-light transmitting electrode layer on a support body (transparent glass substrate) and has also proposed that the reading-light transmitting electrode layer is used as a positive electrode and that comb teeth are formed by the semiconductor fabrication technology in sufficiently narrow intervals of a fine pitch between comb teeth corresponding to a pixel pitch. That is, it has been proposed that the electrode of the reading-light transmitting electrode layer is constructed of a stripe electrode consisting of electrode elements disposed in the intervals of a pixel pitch. In this case, the stripe electrode is first formed on the transparent glass substrate by photoetching, etc. Then, the photoconductive reading layer, the photoconductive recording layer, the charge transfer layer, the photoconductive reading layer, and the recording-light transmitting electrode layer are formed in sequence. Although a specific numeral value for the pixel pitch has not been indicated, it would be obvious to those skilled in this field that the pixel pitch is 50 to 200 xcexcm, because in the medical X-ray photographing, the image recording medium maintains high sharpness and makes a high S/N ratio possible.
In addition, the aforementioned Japanese Patent Application No. 10(1998)-232824, as with the aforementioned U.S. Pat. No. 4,535,468, has proposed that the S/N-ratio degradation due to the direct injection of the positive charge in the reading-light transmitting electrode layer can be prevented by providing a blocking layer of about 500 xc3x85, which consists of an organic substance such as CeO2, between the reading-light transmitting electrode layer and the photoconductive reading layer.
On the other hand, the inventors of this application have made various investigations with respect to the image recording medium proposed in the aforementioned Japanese Patent Application No. 10(1998)-232824 and found the following facts:
(1) The method, which forms an ITO film of thickness 50 to 200 nm (i.e., the reading-light transmitting electrode layer) on the transparent glass substrate and then forms the stripe electrode by photoetching, is preferred because it can form a fine stripe pattern inexpensively;
(2) The photoconductive recording layer has high dark resistance, if it is constructed of an xcex1-Se layer having a thickness of 50 to 1000 xcexcm;
(3) The charge transfer layer is excellent in after image and read response speed, if it is a stacked hole transfer layer consisting of two stacked layers: a first hole transfer layer of thickness 0.1 to 1 xcexcm, consisting of a thin organic substance, which has electrons to form a charge storage portions, and a second hole transfer layer of thickness 5 to 30 xcexcm consisting of xcex1-Se doped 10 to 200 ppm with Cl, which transfers a hole at high speed and has a small number of hole traps;
(4) The photoconductive reading layer has high dark resistance, if it is constructed of an xcex1-Se layer having a thickness of 0.05 to 0.5 xcexcm; and
(5) If the charge transfer layer is a stacked hole transfer layer constructed of a first hole transfer layer of thickness 0.1 to 1 xcexcm consisting of PVK or TPD and a second hole transfer layer of thickness 5 to 30 xcexcm having xcex1-Se doped 10 to 200 ppm with Cl as its main component, then the first charge transfer layer functions a strong insulator with respect to a latent-image polarity charge and the second charge transfer layer transfers a transfer polarity charge at high speed. This makes the charge transfer layer an ideal charge transfer layer which is excellent in afterimage and read response speed. However, even in the case where the second hole transfer layer of the charge transfer layer is replaced with xcex1-Se of thickness 5 to 30 xcexcm so that the charge transfer layer also functions as the photoconductive reading layer, relatively satisfactory effects are obtained and fabrication becomes easy.
From the foregoing facts it is found that the image recording medium described in the aforementioned Japanese Patent Application No. 10(1998)-232824 is a multi-layer recording medium which has high dark resistance and excellent read response time. It is desirable that the image recording medium be constructed mainly of a layer having xcex1-Se as its main component. The glass substrate employs, for example, Corning-1737 (thickness 1.1 mm) made by Corning, and the effective size of the medium is, for example, 20xc3x9720 cm or more. When it is used in chest photographing, the effective size is 43xc3x9743 cm.
The above-mentioned image recording medium, incidentally, is subjected to a great temperature fluctuation cycle during shipping transport under a severe environment, for example, a cold climate condition. If the image recording medium with the selenium (Se) multi-layer film on the glass substrate, among the image recording media disclosed in the aforementioned Japanese Patent Application No. 10(1998)-232824, is subjected to such a great temperature fluctuation cycle, thermal stress will occur at the interface between the glass substrate and the selenium (Se) film (i.e., the photoconductive layer) because of a difference in thermal expansion coefficient therebetween (approximately 10 times). As a result, the problem of destruction due to a difference in thermal expansion will arise. For instance, the selenium (Se) film and the glass substrate are separated from each other, or the selenium (Se) film breaks, or the glass substrate cracks. The larger the image recording medium, the more conspicuous the problem of destruction.
As described above, the image recording medium is subjected to a great temperature fluctuation cycle during ship transport under a severe environment, for example, a cold climate condition. In that case, the image recording medium changes largely to low temperature and then returns to the normal temperature. If the image recording medium with the selenium (Se) multi-layer film on the glass substrate, among the image recording media disclosed in the aforementioned Japanese Patent Application No. 10(1998)-232824, is subjected to such a great temperature cycle, particularly a temperature cycle on the low temperature side, then the problem of destruction due to a difference in thermal expansion will arise, because the thermal expansion coefficient of the glass substrate is about ten times smaller than that of the selenium (Se) film. For example, as shown in FIG. 10A, the thermal contraction amount of the glass substrate becomes much smaller than that of the selenium (Se) film, so that the surface of the selenium (Se) film cannot endure tensile stress and breaks. In addition, the selenium (Se) film and the glass substrate are separated from each other (FIG. 10C). Furthermore, since the glass substrate is a brittle material which does not deform flexibly (tends to crack), the glass substrate cracks (FIG. 10D). The larger the image recording medium, the more conspicuous the problem of destruction.
The present invention has been made in view of the above-mentioned circumstances. Accordingly, it is an object of the present invention to provide an image recording medium that is capable of eliminating the problem of destruction due to a difference in thermal expansion even when the selenium (Se) film is formed on the support body. Another object of the invention is to provide an image recording medium which is capable of eliminating the problem of destruction resulting from a fall in temperature, even when the selenium (Se) film is formed on the support body.
In accordance with an important aspect of the present invention, there is provided a first image recording medium comprising: a support body which allows reading electromagnetic waves to pass through it; a first electrode layer, formed on the support body, which allows the reading electromagnetic waves to pass through it; a photoconductive reading layer, formed on the first electrode layer, which exhibits conductivity when irradiated with the reading electromagnetic waves; a charge storage portion, formed on the photoconductive reading layer, for storing a latent-image polarity charge; a photoconductive recording layer, formed on the charge storage portion, for generating the latent-image polarity charge when irradiated with recording electromagnetic waves; and a second electrode layer, formed on the photoconductive recording layer, which allows the recording electromagnetic waves to pass through it; wherein a difference in thermal expansion coefficient between the support body and the photoconductive reading layer is small so that no structural destruction occurs between the support body and the photoconductive reading layer because of thermal stress generated by a temperature change.
The expression xe2x80x9cno structural destruction occurs between the support body and the photoconductive reading layerxe2x80x9d not only means that no structural destruction occurs between the two layers, but also means that neither the support body nor the photoconductive reading layer is structurally destructed.
In the first image recording medium of the present invention, it is desirable that the support body be deformable with a temperature change. Particularly, it is desirable that the support body be deformable when the temperature of the surrounding environment falls.
The critical point at which structural destruction occurs because of thermal stress depends on the materials of the support body and the photoconductive reading layer. It is desirable that the thermal expansion coefficient of the support body be within a few tenths to a few times the thermal expansion coefficient of the photoconductive reading layer in order to prevent structural destruction. It is more desirable that the thermal expansion coefficients of the support body and the photoconductive reading layer be approximately equal and that a difference in thermal expansion between the two be within 2.5xc3x9710{circumflex over ( )}xe2x88x925/K@40xc2x0 C. where xe2x80x9c{circumflex over ( )}xe2x80x9d indicates an exponent. It is most desirable that the thermal expansion coefficients of the two be nearly the same.
More specifically, in the case where the photoconductive reading layer has amorphous selenium (xcex1-Se) as its main component, the thermal expansion coefficient of the selenium (Se) material is 3.68xc3x9710{circumflex over ( )}xe2x88x925/K@40xc2x0 C. Therefore, it is preferable that the thermal expansion coefficient of the support body be 1.2 to 6.2xc3x9710{circumflex over ( )}xe2x88x925/K@40xc2x0 C. and further preferable that it be is 2.2 to 5.2xc3x9710{circumflex over ( )}xe2x88x925/K@40xc2x0 C. 
The material of the support body, having a thermal expansion coefficient within the aforementioned range, and deformable with a temperature change, can employ an organic polymer material, such as polycarbonate, polymethylmethacylate (PMMA), etc.
In accordance with another important aspect of the present invention, there is provided a second image recording medium comprising: a support body which allows reading electromagnetic waves to pass through it; a first electrode layer, formed on the support body, which allows the reading electromagnetic waves to pass through it; a photoconductive reading layer, formed on the first electrode layer, which exhibits conductivity when irradiated with the reading electromagnetic waves; a charge storage portion, formed on the photoconductive reading layer, for storing a latent-image polarity charge; a photoconductive recording layer, formed on the charge storage portion, for generating the latent-image polarity charge when irradiated with recording electromagnetic waves; and a second electrode layer, formed on the photoconductive recording layer, which allows the recording electromagnetic waves to pass through it; wherein between the photoconductive reading layer and the first electrode layer, there is provided a buffer layer, which allows the reading electromagnetic waves to pass through it, for alleviating thermal stress between the photoconductive reading layer and the first electrode layer.
Preferably, the buffer layer, in addition to being a layer which alleviates thermal stress, has both the performance of blocking the electric charge that is injected via the electrode of the first electrode layer and the function of suppressing interfacial crystallization between the first electrode layer and the photoconductive reading layer. Furthermore, it is preferable that the buffer layer be a layer which reinforces the connection between the first electrode layer and the photoconductive reading layer.
The expression xe2x80x9calleviates thermal stressxe2x80x9d is intended to mean that a thermal expansion mismatch is alleviated so that the aforementioned structural destruction will not occur because of a difference in thermal expansion coefficient between the photoconductive reading layer and the support body. More specifically, it is necessary that the buffer layer allow transmission of light and have the blocking performance and further necessary that it be elastic. For this reason, it is desirable that the buffer layer be constructed of an organic thin film, which is transparent and elastic and has satisfactory blocking performance, such as organic polymer insulation materials (e.g., polyamide, polyimide, polyester, polyvinylbutyral, polyvinylpyrolidone, polyurethane, polymethylmethacrylate, polycarbonate, etc.), a mixed film, consisting of an organic binder and a low molecular organic material, etc.
It is preferable that the thickness of the buffer film be about 0.05 to 5 xcexcm. However, a range of 0.1 to 5 xcexcm is preferred in order to buffer thermal stress. On the other hand, a range of 0.05 to 0.5 xcexcm is preferred in order to obtain satisfactory blocking performance having no afterimage. From a trade-off between the two, it is desirable that the film thickness be in a range of 0.1 to 0.5 xcexcm.
According the first image recording medium of the present invention, the difference in thermal expansion coefficient between the support body and the photoconductive reading layer is small so that no structural destruction occurs between the support body and the photoconductive reading layer because of thermal stress generated by a temperature change in environment. Thus, the first image recording medium of the present invention is capable of eliminating the problem of structural destruction which is caused by thermal stress resulting from the difference in thermal expansion coefficient between the support body and the photoconductive reading layer.
For example, in the case where the temperature of environment falls, there is no possibility that the thermal contraction amount of the photoconductive reading layer (e.g., the selenium (Se) film) will become greater than that of the support body, if the thermal expansion coefficient of the support body is approximately equal to the thermal expansion coefficient of the photoconductive reading layer, or within several times. Since tensile force is not exerted on the surface of the photoconductive film as is done in the prior art, the photoconductive film is less likely to break. On the other hand, if the thermal expansion coefficient of the support body is greater than that of the photoconductive reading layer, compressive stress will be exerted on the photoconductive film. However, if a difference in thermal expansion coefficient between the support body and the photoconductive film is within several times, destruction such as breakage or separation of the photoconductive film will not occur, because the photoconductive film has high resistance to compressive stress, like ordinary brittle materials.
Even in the case where the thermal expansion coefficient of the support body is great and therefore a difference in thermal contraction amount occurs between the support body and the photoconductive film, if the support body is deformable with a temperature change in environment, the image recording medium absorbs the different in thermal contraction amount by flexibly deforming so that the center of curvature is moved to the side of the support body, for example, when the temperature of environment falls. Thus, the support body is less liable to crack.
Moreover, since compressive stress is exerted on the entire surface of the photoconductive film (selenium film), the magnitude of tensile stress in the circumferential portion of the photoconductive film is alleviated even when the photoconductive film is deformed so that the center of curvature is moved to the side of the support body. Thus, the photoconductive film is also less liable to break even when the aforementioned deformation occurs. When the thermal expansion coefficients of the support body and the photoconductive reading layer are nearly the same, a problem which is caused by the influence of thermal stress resulting from the difference in thermal expansion coefficient between the support body and the photoconductive reading layer will not arise. If the support body employs an organic polymer material, the support body has the advantage that it has a strong resistance to shock, compared with a glass substrate.
Furthermore, the second image recording medium of the present invention is constructed so that, between the photoconductive reading layer and the first electrode layer, there is provided a buffer layer, which allows the reading electromagnetic waves to pass through it, for alleviating thermal stress between the photoconductive reading layer and the first electrode layer. Thus, the second image recording medium is capable of alleviating a thermal expansion mismatch between the two layers. Even in the case where the difference in thermal expansion coefficient between the two layers is great (e.g., about 10 times, the aforementioned structural destruction can be prevented. This makes it possible to freely select the material of the support body independently of the thermal expansion coefficient of the photoconductive reading layer.